(1) Field of the Invention
The present invention relates to an interpolative digital-to-analog (D/A) converter, and more particularly to a D/A converter by which the digital signal of signals sampled at a sampling frequency several times higher than the Nyquist rate of the signals is converted into an analog signal. Especially, it relates to a D/A converter for a voice band signals etc. which is well suited for implementation as a semiconductor integrated circuit.
(2) Description of the Prior Art
As one system of a D/A converter of high precision for handling voice signals, there has been known an oversampling type D/A converter which is, in principle, capable of reducing the number of analog circuit elements and mitigating precisions to be required of constituent elements and which is well suited to be implemented by a semiconductor integrated circuit. However, as described in Report of the Institute of Electronics and Communication Engineering in Japan (CS83-198), National Conference Record No. 659, 1984 Communications' etc., the oversampling type D/A converter in a prior art needs a sampling frequency higher than several MHz in order to attain a desired S/N (signal-to-noise) ratio characteristic (e. g., 90 dB for an evaluation band of 4 kHz), so that high-speed operations are required of an operational amplifier, an analog switch etc. Moreover, since a difference PCM (pulse code modulation) signal is used as a digital signal being an input to a D/A converter, an analog integrator is necessitated on the output side of the D/A converter, and the analog integrator forms causes for spike noise and an offset voltage to worsen the S/N ratio, so that a result contradictory to the original purpose of improving the S/N ratio is produced.